The present invention relates to traction modifying differential devices, and more particularly, to such devices of the type in which the differential gearing can be “locked” in response to an input signal, such as an electrical input signal.
Traction modifying locking differentials of the type to which the present invention relates typically include a gear case defining a gear chamber, and disposed therein, a differential gear set including at least one input pinion gear and a pair of output side gears. Typically, such a “locking differential” includes some sort of locking mechanism to prevent rotation of one of the side gears relative to the gear case, the engagement of the locking mechanism being initiated by some sort of actuator. By way of example only, the actuator could include a ball ramp mechanism in which rotation of a ramp plate is retarded relative to the gear case, thus initiating ramping, in response to a signal being transmitted to an electromagnetic coil disposed adjacent the ramp plate. Examples of locking differentials of the type described hereinabove are shown in U.S. Pat. Nos. 6,083,134 and 6,551,209, both of which are assigned to the assignee of the present invention and incorporated herein by reference.
In both of the locking differentials illustrated and described in the above-incorporated patents, transmitting an appropriate input signal to the electromagnetic coil results in a locking member engaging a mating portion associated with the differential side gear disposed immediately adjacent the actuator arrangement (which includes the electromagnetic coil). Although electrically actuated locking differentials made in accordance with the teachings of the above-incorporated patents are now in commercial production, and are performing in a satisfactory manner, the design configuration illustrated and described in the above-incorporated patents has one inherent characteristic which can limit the performance capability of the locking differential. With the actuator utilizing a conventional ball-ramp device (i.e., one having two ramp plates with a set of cam balls being disposed therebetween), a substantial amount of packaging space, both radially and axially, is taken up just by the ball ramp device. Furthermore, ball ramp devices inherently have a relatively small axial displacement, as the device “ramps up”, and that can represent a disadvantage in a locking differential, as will be described subsequently.
As should be understood by those skilled in the art, the torque capacity of the locking mechanism in a locking differential is a function of (and is generally proportional to) the axial travel of the locking mechanism, as it moves between the locked and unlocked conditions. Furthermore, the travel of the locking mechanism is, in the devices of the above-incorporated patents, limited by the axial travel of the ball ramp device, i.e., by the axial travel of the “inner ramp plate” disposed adjacent the differential case. It has now also become understood that in the devices of the above-incorporated patents, the travel of the locking mechanism, and therefore, the torque capacity of the locking mechanism is further limited by the arrangement wherein the locking mechanism is associated with the side gear adjacent the actuator (the electromagnetic coil and ball ramp device), such that there is within the “package” a limited amount of space for the locking mechanism to achieve its “travel”. By way of example only, in a commercial embodiment of a locking differential made in accordance with the teachings of above-incorporated U.S. Pat. No. 6,551,209, the locking members have an axial travel of only about 4 mm., and therefore, an effective “locking engagement” of only about 3 mm.
Also, as is known by those skilled in the art, the travel of the locking mechanism (in moving from the unlocked to the locked condition) must be matched by a biasing spring arrangement capable of moving the locking mechanism over a distance equal to the “travel”, but oppositely, from the locked condition to the unlocked condition. The requirement for a biasing spring arrangement which is able to move the locking mechanism a distance equal to the required “travel” of the locking mechanism makes it much more difficult to package the locking mechanism, especially in the case of those devices in which the locking mechanism is on the end of the device immediately adjacent the actuator.